La Crosse (LAC) encephalitis virus is an important mosquito-borne agent occurring throughout the eastern half of the United States. Feral populations of the vector, Aedes triseriatus, breed primarily in small water-containing tree holes, but the mosquito has adapted to discarded auto tires and other small man-made water containers. The continuing long-term objective of this project is to determine and quantify the factors that influence the epidemiology of the disease. The current working hypothesis is that vertebrate amplification is not essential for LAC virus survival in localities where the Aedes triseriatus population has a high maternal vertical transmission rate (MVTR), i.e., 60% or higher. Venereal transmission is the probable compensating mechanism for virus erosion during vertical transmission. Virus survival becomes increasingly dependent on vertebrate amplification as the MVTR decreases. Two current major objectives include determining venereal transmission rates under various mating/bloodfeeing schedules, and follow-up on preliminary results indicating that Ae. triseriatus that feed on deer blood containing Jamestown Canyon (JC) virus N antibodies lose their ability to transmit LAC virus. Deer are a major blood meal source of Aedes triseriatus and such an interference phenomenon could be a major factor explaining the discontinuous "pocket" distribution of clinical La Crosse encephalitis. Another major objective is to determine LAC virus transmission thresholds and to devise control strategies that will reduce feral Ae. triseriatus populations to levels sufficiently low that transmission to amplifiers is interrupted. Insecticide treatment of natural oviposition sites supplemented if necessary be treated ovitraps is the first approach to be employed. All of these investigations will require only more or less standard entomological and virological methods. Other main objectives are the development of an improved double antibody sandwich enzyme-linked immunoassay for rapid LACV isolation from individual mosquitoes, and adapting the cuticular growth bands method for determining the calendar age of individual Ae. triseriatus, The former will greatly facilitate virus assay in may kinds of experiments and the latter will provide invaluable insights on Ae. triseriatus field behavior during the critical first 12-14 days of adult life. The cuticular aging method, if successful, should provide a new level of confidence in vector population models.